In the related art, for example, Patent Documents 1 and 2 disclose a technique for determining degradation of a secondary battery provided in a vehicle. Patent Document 1 discloses a technique which measures an open-circuit voltage OCV and a minimum voltage Vst when the engine starts and detects the degradation of the secondary battery using a characteristic map defining the relation between OCV and Vst. The characteristic map is divided into five regions corresponding to the levels of OCV and Vst and the boundary between the regions is divided by a logarithmic curve or a straight line.
Patent Document 2 discloses a technique which determines the degradation of the secondary battery using a map indicating the relation between internal resistance R and an open-circuit voltage OCV corresponding to the degree of degradation of the secondary battery. This technique measures a current and a voltage before the engine starts and a current and a voltage when the integrated value of a discharge current after the engine starts reaches predetermined quantity of electricity and calculates the internal resistance R using these measured values.